1. Field of the Invention
This invention relates to a method for generating a radiation image signal wherein an image signal is obtained by reading out a radiation image which has been recorded on a recording medium during an image recording operation using a grid and which comprises an object image and a striped grid image corresponding to the grid and superposed upon the object image. This invention also relates to an image processing method for such a radiation image. This invention further relates to a radiation image read-out apparatus wherein a method for generating a radiation image signal is employed.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on a sheet of X-ray film having a small gamma value designed for the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor varies over a wide range and is proportional to the amount of energy stored during exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain a desirable image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material or a display device.
During the recording of a radiation image of an object on a recording medium, such as X-ray film or a stimulable phosphor sheet, a grid is often located between the object and the recording medium such that radiation scattered by the object does not impinge upon the recording medium. The grid is constituted of bars of a radiation-impermeable material, such as lead, and bars of a radiation-permeable material, such as aluminium or wood, which are alternately located in parallel at small pitches of approximately 4.0 bars/mm. When the grid is used during the recording of a radiation image of an object on a recording medium, radiation scattered by the object is prevented from impinging upon the recording medium, and therefore the contrast of the radiation image of the object can be kept high. However, a grid image having a striped pattern is recorded together with the object image on the recording medium.
In general, in radiation image read-out apparatuses, wherein an image signal is detected from a recording medium which has a radiation image recorded thereon, light which is emitted from the recording medium and which carries information about the radiation image is photoelectrically detected and converted into an image signal. The image signal is then sampled at sampling intervals of .DELTA.x=1/(2.multidot.fss) corresponding to the spatial frequency, which is the maximum of a spatial frequency range necessary for image information. The spatial frequency, which is the maximum of a spatial frequency range necessary for image information, is herein denoted by fss. The sampled image signal is then digitized. In cases where the radiation image comprises the object image and a grid image superposed upon the object image, the image signal obtained in the manner described above includes not only the information representing the radiation image of the object but also noise which is caused to occur by the grid image. The noise will occur even if the spatial frequency of the grid image is higher than the maximum spatial frequency fss necessary for image information.
FIG. 6A is a graph showing the spatial frequency characteristics of a radiation image, which has been recorded on a recording medium and which comprises an object image and a grid image superposed upon the object image, along a direction intersecting perpendicularly to the striped pattern of the grid image.
By way of example, it is herein assumed that, during the recording of the radiation image, a grid having the bars of a radiation-impermeable material and the bars of a radiation-permeable material, which are alternately located in parallel at pitches of 4.0 bars/mm, was used. The spatial frequency of the grid image is 4 cycles/mm. Also, it is assumed herein that the spatial frequency fss, which is the maximum of a spatial frequency range necessary for the reproduction of a visible radiation image of the object, is 2.5 cycles/mm.
FIG. 6B is an explanatory graph showing how noise occurs when an image signal is sampled at sampling intervals of .DELTA.x=1/(2.multidot. fss)= 0.2 (mm) corresponding to the spatial frequency fss=2.5 (cycles/mm), i.e. is sampled five times per mm. When such sampling intervals are applied, it is possible to obtain information in the spatial frequency region which is below the spatial frequency fss=2.5 (cycles/mm), which is the maximum of a spatial frequency range necessary for the reproduction of a visible radiation image of the object.
In FIG. 6B, the same curve as that shown in FIG. 6A is indicated by the solid line. As indicated by the broken line, noise occurs at the position corresponding to 1 cycle/mm, with which the position of the peak occurring at 4 cycles/mm coincides when the curve indicated by the solid line is folded back from the part corresponding to fss=2.5 (cycles/mm). Such noise is referred to as aliasing. Specifically, the aliasing corresponding to a spatial frequency of 4 cycles/mm of the grid image occurs at the position corresponding to 1 cycle/mm.
FIG. 6C is a graph showing the spatial frequency characteristics of the radiation image represented by an image signal obtained from the sampling in which sampling intervals of .DELTA.x=1/(2.multidot.fss)= 0.2 (mm) are applied.
In cases where a visible image is reproduced to a scale of one to one from the image signal, which includes the noise corresponding to the grid image and occurring at the position of 1 cycle/mm, a striped pattern having a spatial frequency of 1 cycle/mm occurs on the reproduced visible image. Even if the spatial frequency of the grid image falls within such a spatial frequency range that the grid image is not very perceptible, when an image signal is sampled, noise which constitutes a striped pattern will occur in such a spatial frequency range that the grid image is perceptible. When the sampled image signal is used in the reproduction of a visible image, a visible image having bad image quality is obtained.